In the recovery of oil from oil-bearing subterranean formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. The most widely used supplemental recovery technique is water flooding which involves the injection of water into the formation. As the water moves through the formation, it acts to drive oil therein toward a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that the chemistry of various waters encountered in oil field operations is such that low solubility compounds are present. Under certain conditions, these sparingly soluble salts may precipitate out and lead to the formation of scale, i.e. hard solid deposits, on walls of pipework, heat exchanger surfaces, valves, vessels and elsewhere in the oil recovery system. Also, the formation of insoluble salts will tend to plug the oil-bearing strata and reduce the chances of obtaining an improvement in the recovery of oil. The problems associated with scale formation in oil production are described in: K. S. Johnson, "Water Scaling Problems in the Oil Production Industry", Soc.Chem., Spec.-Publ.-R. 1983 (45), pp.125-149.
Oil field scales most commonly contain the carbonate and sulphate salts of alkaline earth metals such as calcium, barium and strontium, and often include calcium carbonate (calcite) and barium sulphate (barite). The carbonate scales are most likely to precipitate from formation water (i.e., water that is present within the rock formation) as the pressure drops during production allowing carbon dioxide dissolved in the brine to be released and the pH to increase resulting in carbonate ion formation.
In oil recovery processes where seawater which contains high concentrations of sulfate ion is injected during secondary recovery procedures, the formation water mixes with seawater. Such mixing of incompatible waters can lead to severe scaling problems throughout the production system. This is likely to occur downhole when seawater breakthrough takes place in production wells, and at the surface when fluids from wells producing formation water alone are mixed with fluids from wells which have separate seawater breakthrough. In certain subterranean formations, e.g. those found in the North Sea, the formation water tends to have a low pH and high barium content. In such formations, a rapid deposition of barite will occur at the seawater-formation water interface.
In the production of fluids from formations which are susceptible to such scale formation, the production rates tend to decline steadily as the scale forms. To restore the production rates from such formations, various methods have been used.
In one method, the formations are re-perforated by opening new perforations through the well casings and exposing new formation surfaces. This method can be used to temporarily restore production rates, but is subject to further plugging of the formation by additional scale. Also, this method can be relatively expensive and is therefore of limited value in formations where rapid scale deposition occurs.
A second method involves the use of acid to remove scale buildup. While the use of acid treatments is effective in many instances, it does require removal of the well from production for the acid treatment process which is disadvantageous especially if the formation is subject to rapid scale deposition. In addition, the production rate begins to decline after the treatment as more scale is formed so that during much of its producing life the well is producing fluids at a reduced rate. Further, certain scales, such as barite, can not be removed even with conventional oil field acid treatments.
Barite scale represents a particular problem among the mineral scalants due to its extremely low solubility and speed of precipitation. A few methods are available for dissolving barite, but they are very expensive both in terms of the large volumes of fluid circulation and in the down time involved in carrying out a descaling operation. In most instances the barite cannot be removed which results in the additional cost of replacing the scaled-up equipment. When it is not possible to replace scaled oil field equipment that is permanently fixed in a producing well, the formation must be re-perforated as discussed above. Thus, treatment of barite and other scale has focussed on the development and use of scale inhibitors.
It has been proposed to add scale inhibitors to the flood water during water injection and also to topside production systems. Scale inhibitors have also been used for treating scaling problems which often occur at the well bottom or as produced fluids progress up a production well. The only practical method of getting scale inhibitor into these fluids is by the so-called "squeeze" operation. The squeezing technique consists of reverse flowing a production well by applying excess pressure from the surface facility and adding the scale inhibitor to the reverse flow.
In order to be cost-effective, a scale inhibitor must meet various criteria. A good scale inhibitor should either: (1) retard the precipitation of sparingly soluble salts; or (2) modify the properties of the scale crystals (e.g., their shapes and tendencies to disperse) to reduce crystal adherence on walls and to facilitate their disposal as sludge. In addition, to be suitable for oil field water applications, a scale inhibitor should efficiently inhibit scale formation in environments characterized by high temperature, low pH and high concentrations of divalent and trivalent metal ions (i.e., high ionic strength). A cost-effective scale inhibitor should also be effective at a concentration within the range of 10 to about 500 ppm.
Prior to the present invention, scale inhibitors have been developed and used with varying degrees of success to inhibit scale during oil field operations. Examples of such inhibitors which are commercially available include "Bellasol S-29/S-40", (a partial sodium salt of a polyacrylic acid from Ciba-Geigy), and "QR-980" (a partial sodium salt of a copolymer of an acrylate and methacrylic acid available from Rohm & Haas). These inhibitors have not been completely satisfactory in their performance or cost. Certain inhibitors are not effective at low use levels, in low pH, or high temperature environments. Moreover, none have been found to be satisfactory for inhibiting barium sulphate scale, when the barium ion concentration is greater than about 500 ppm.
There have been a number of attempts to develop scale inhibitors for barium sulphate which operate in low pH, high temperature and high salinity environments. For example, N. C. van der Leeden et al., "Development of Inhibitors for Barium Sulphate Deposition", Proceedings of the Third International Symposium on Chemicals in the Oil Industry. (1988) 65-84, disclosed that introduction of sulphonated vinyl segments in a polymaleic based polymer would increase the effectiveness at low pH values. Also, N. C. van der Leeden et al., "Inhibition of Barium Sulphate Deposition by Polycarboxylates of Various Molecular Structures" Society of Petroleum Engineers: Production Engineering, (Feb. 1990) 70-76, disclosed a study of the relationship between the molecular structure of polycarboxylates and their growth-retarding influence on barium sulphate. Two types of polycarboxylates with a molecular structure based on either polyacrylic or maleic acid were studied.
Copolymers of maleic anhydride and acrylamide or methacrylamide, and terpolymers comprising maleic anhydride, acrylamide or methacrylamide and other copolymerizable monomers have been suggested as scale control agents. Examples of some publications describing such polymers and their use as scale control agents include U.S. Pat. Nos. 4,065,607 and 4,126,549; UK Patent 1,519,512; and EP 297,049. European patent application 297,049 describes polymers based on vinyl sulfonates which are reportedly useful in inhibiting barium sulfate scale deposition from aqueous systems. The comonomers which may be reacted with the vinyl sulfonate include maleic acid or maleic anhydride and/or comonomers such as acrylamide, N,N-dimethylacrylamide, styrene, etc.
There continues to be a need for a cost-effective scale reducing systems that are effective for inhibiting barium sulphate and other scale from oil production systems and subterranean formations with environments characterized by high temperature, high concentrations of divalent and trivalent metal ions, and low pH.